As known, these suspension bridges have vibration frequencies of their own; normally, with no wind, the basic flexural vibration frequency differs from the basic torsional vibration frequency, both being generally very low. Nevertheless, the action of side winds varies said typical vibration frequencies, particularly because--especially in bridges with large transversal dimensions and/or a wide span, for instance motorway bridges--the flat suspended structure behaves, when actually exposed to side winds, similarly to a wing surface, hence with a "lifting" effect which greatly varies from one moment to the next.
As wind increases its strength, the two aforespecified vibration frequencies tend to approach, up to the point of coinciding: in these circumstances, the structure is thus subjected to so-called "flutter" conditions, i.e. to flexural-torsional stresses which may even result dangerous for the stability of the whole bridge.
These flutter conditions, and the problems connected therewith, are described in more detail in EP-A-0.233.528, filed by the same Applicant, to which reference is made for a better understanding of the present invention.
The main object of EP-A-0.233.528 is a wing structure, which is rigidly fixed to the lateral edges of the bridge and is meant to increase its flutter speed beyond the top speed of the wind expected in the bridge area.
The same EP-A-0.233.528 also discloses a windbreak barrier, whose main function is to reduce the crosswind pressure on the vehicles travelling along the bridge; such a barrier essentially consists of a grating, having a typically convex surface apt to cooperate with the wing structure, so as to control the action of the wind.